U.S. patent number 8,957,564 [Application Number 13/172,514] was granted by the patent office on 2015-02-17 for microelectromechanical system megasonic transducer.
This patent grant is currently assigned to Silicon Light Machines Corporation. The grantee listed for this patent is Zarem Harold, Toshio Hiroe, James Hunter, Alexander Payne. Invention is credited to Zarem Harold, Toshio Hiroe, James Hunter, Alexander Payne.
United States Patent |
8,957,564 |
Hiroe , et al. |
February 17, 2015 |
Microelectromechanical system megasonic transducer
Abstract
Megasonic cleaning systems and methods of fabricating and using
the same are provided. In one embodiment, the system comprises a
plurality of Micro-Electromechanical System (MEMS) transducers,
each transducer including a movable membrane with a membrane
electrode coupled to a first potential disposed above and spaced
apart from an upper surface of a die including a cavity electrode
coupled to a second potential, the membrane including multiple
layers including a polysilicon layer between a top silicon nitride
layer and a bottom silicon nitride layer, and the membrane
electrode includes the polysilicon layer; a chuck on which a target
workpiece is positioned; and a fluid to couple sonic energy from
the plurality of MEMS transducers to the target workpiece. Other
embodiments are also provided.
Inventors: |
Hiroe; Toshio (Hazukashi,
JP), Harold; Zarem (Palo Alto, CA), Payne;
Alexander (Ben Lomond, CA), Hunter; James (Campbell,
CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Hiroe; Toshio
Harold; Zarem
Payne; Alexander
Hunter; James |
Hazukashi
Palo Alto
Ben Lomond
Campbell |
N/A
CA
CA
CA |
JP
US
US
US |
|
|
Assignee: |
Silicon Light Machines
Corporation (Sunnyvale, CA)
|
Family
ID: |
52463615 |
Appl.
No.: |
13/172,514 |
Filed: |
June 29, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
61359519 |
Jun 29, 2010 |
|
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Current U.S.
Class: |
310/309; 134/1.2;
134/1; 134/1.3 |
Current CPC
Class: |
B06B
1/0292 (20130101); B08B 3/12 (20130101) |
Current International
Class: |
H02N
2/00 (20060101); B08B 3/12 (20060101) |
Field of
Search: |
;310/309 ;134/1-1.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dougherty; Thomas
Attorney, Agent or Firm: Nuttle; William
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims the benefit of priority under 35
U.S.C. 119(e) to U.S. Provisional Patent Application Ser. No.
61/359,519 entitled "Microelectromechanical System Megasonic
Transducer," filed Jun. 29, 2010, which application is hereby
incorporated by reference in its entirety.
Claims
What is claimed is:
1. A system comprising: a plurality of Micro-Electromechanical
System (MEMS) transducers, each transducer including a movable
membrane with a membrane electrode coupled to a first potential
disposed above and spaced apart from an upper surface of a die
including a cavity electrode coupled to a second potential, the
membrane including multiple layers including a polysilicon layer
between a top silicon nitride layer and a bottom silicon nitride
layer, and the membrane electrode includes the polysilicon layer; a
chuck on which a target workpiece is positioned; and a fluid to
couple sonic energy from the plurality of MEMS transducers to the
target workpiece.
2. The system of claim 1, further comprising a driver adapted to
apply a drive voltage with a variable phase delay in the drive
voltage applied to a first number of the plurality of MEMS
transducers from that supplied to a second number of MEMS
transducers to focus sonic energy emitted from the plurality of
MEMS transducers.
3. The system of claim 1, wherein the die is attached to a surface
of a printed circuit board and the plurality of MEMS transducers
are electrically coupled to a driver attached to the circuit board,
and further comprising a mechanical backing plate attached to a
back surface of the circuit board opposite the surface to which the
die is attached.
4. The system of claim 3, wherein the driver is attached to the
back surface of the circuit board and enclosed by the mechanical
backing plate.
5. The system of claim 3, wherein the membrane electrodes are
coupled to the first potential through an interconnect grid on the
upper surface of the die, and wherein the cavity electrodes are
coupled to the second potential through electrical connections to
pads on a backside of the die.
6. The system of claim 5, wherein the interconnect grid on the
upper surface of the die is separated from the cavity electrodes in
the die by an oxide layer at least 1 .mu.m thick to reduce coupling
capacitance there between.
7. The system of claim 6, wherein the interconnect grid covers less
than 25% of the upper surface of the die.
8. The system of claim 3, further comprising a reservoir containing
the fluid in which the die and the target workpiece is exposed, and
further comprising a mechanism to provide relative motion between
the target workpiece and the plurality of MEMS transducers on the
die.
9. The system of claim 1, further comprising a nozzle to direct the
fluid to the target workpiece, and wherein the sonic energy is
coupled to the fluid in the nozzle.
10. The system of claim 1, wherein the movable membrane comprises a
substantially circular surface, and wherein a diameter of the
circular surface is selected to provide the MEMS transducers with a
megasonic resonant frequency of from about 3 to about 10 megahertz
(MHz).
11. The system of claim 10, wherein the circular surface has a
diameter of from about 3 to about 100 micrometers (.mu.m).
12. A system comprising: a plurality of Micro-Electromechanical
System (MEMS) transducers, each transducer including a movable
membrane with a membrane electrode coupled to a first potential
disposed above and spaced apart from an upper surface of a die
including a cavity electrode coupled to a second potential, the
membrane including multiple layers including a polysilicon layer
between a top silicon nitride layer and a bottom silicon nitride
layer, and the membrane electrode includes the polysilicon layer;
an interconnect grid on the upper surface of the die through which
the membrane electrodes are coupled to the first potential; a
circuit board including a surface to which the die is attached; a
driver attached to the circuit board and electrically coupled to
the plurality of MEMS transducers to apply a drive voltage between
the membrane electrodes and cavity electrodes to operate the
plurality of MEMS transducers at a megasonic resonant frequency;
and a chuck on which a target workpiece is positioned.
13. The system of claim 12, wherein the driver is adapted to apply
a drive voltage with a variable phase delay in the drive voltage
applied to a first number of the plurality of MEMS transducers from
that supplied to a second number of MEMS transducers to sweep sonic
energy emitted from the plurality of MEMS transducers over a
surface of the target workpiece.
Description
TECHNICAL FIELD
The present invention relates generally to sonic transducers, and
more particularly to megasonic systems including
MicroElectroMechanical System (MEMS) transducers and to methods of
fabricating and using the same.
BACKGROUND
Sonic transducers are widely used for a number of applications
including medical imaging, cleaning systems or scrubbers used in
fabricating semiconductor or Micro-Electromechanical System (MEMS)
devices. In a typical cleaning system substrates, such as silicon
wafers, are immersed in a liquid to which sonic energy is applied.
High intensity sound waves generate pressure fluctuations that lead
to cavitation, a condition in which millions of microscopic bubbles
rapidly form and collapse in the liquid. The collapse of these
cavitation bubbles produce shock waves that impinge on substrate
surfaces, dislodging particles thereon. Conventional cleaning
systems use typically piezoelectric transducers operating at
ultrasonic frequencies of less than about 400 kHz to apply sonic
energy to the liquid. However, as the sizes of elements or features
in semiconductor circuit MEMS devices continues to shrink, the
trend in sonic cleaning systems has been toward transducers capable
of operating at higher frequencies, which produce smaller
cavitation bubbles that increase the cleaning effectiveness, and
provide a more gentle cleaning while reducing probability of damage
to the substrate. Unfortunately, the operating frequency or
resonant frequency of piezoelectric transducers is determined by a
film thickness of the piezoelectric material and is generally
limited to the ultrasonic or low megasonic frequency range.
Accordingly, there is a need for a transducer suitable for use in
cleaning systems and capable of operating over the full megasonic
range.
SUMMARY
Megasonic cleaning systems and methods of fabricating and using the
same are provided.
In one embodiment, the system comprises a plurality of
Micro-Electromechanical System (MEMS) transducers, each transducer
including a movable membrane with a membrane electrode coupled to a
first potential disposed above and spaced apart from an upper
surface of a die including a cavity electrode coupled to a second
potential, the membrane including multiple layers including a
polysilicon layer between a top silicon nitride layer and a bottom
silicon nitride layer, and the membrane electrode includes the
polysilicon layer; a chuck on which a target workpiece is
positioned; and a fluid to couple sonic energy from the plurality
of MEMS transducers to the target workpiece.
In another embodiment, the method comprises: (i) providing a
plurality of MEMS transducers, each transducer including a movable
membrane with a membrane electrode coupled to a first potential
disposed above and spaced apart from an upper surface of a die
including a cavity electrode coupled to a second potential; (ii)
positioning a target workpiece on a chuck; (iii) applying a drive
voltage between the membrane electrodes and cavity electrodes to
operate the plurality of MEMS transducers at a megasonic resonant
frequency; and (iv) coupling sonic energy to the target workpiece
from the plurality of MEMS transducers through a fluid.
Optionally, the system can further include a driver, and applying
the drive voltage can include applying the drive voltage with a
fixed phase delay in the voltage applied between individual
transducers or groups of MEMS transducers to allow focusing or
sweeping of the sonic energy.
BRIEF DESCRIPTION OF THE DRAWINGS
These and various other features of including
MicroElectroMechanical System (MEMS) based megasonic systems and
methods of operating the same will be apparent upon reading of the
following detailed description in conjunction with the accompanying
drawings and the appended claims provided below, where:
FIG. 1 is a schematic block diagram of a megasonic system including
MicroElectroMechanical System (MEMS) based transducer according to
an embodiment of the present disclosure;
FIG. 2 is a schematic block diagram in cross-sectional view of a
MEMS transducer according to an embodiment of the present
disclosure;
FIG. 3 is a top plan view of an array of MEMS transducer according
to an embodiment of the present disclosure;
FIG. 4 is a schematic block diagram in perspective view of a
membrane of a MEMS transducer according to an embodiment of the
present disclosure;
FIG. 5 is a schematic block diagram in cross-sectional view of a
portion of a megasonic system illustrating an electrical connection
of a MEMS die through a via in a circuit board according to an
embodiment of the present disclosure;
FIG. 6 is a schematic block diagram in cross-sectional view of a
portion of a megasonic system illustrating an electrical connection
of a MEMS die to a circuit board through a wire-bond and perimeter
etching according to another embodiment of the present
disclosure;
FIG. 7A is a schematic diagram in plan view of an interconnect grid
for membrane electrodes of a MEMS transducer including according to
an embodiment of the present disclosure;
FIG. 7B is a schematic block diagram in cross-sectional view of the
interconnect grid of FIG. 7A;
FIG. 8 is a schematic diagram in plan view of an interconnect grid
according to an embodiment of the present disclosure;
FIG. 9 is a top plan view of a megasonic system including a circuit
board mounting a plurality of MEMS die according to an embodiment
of the present disclosure;
FIG. 10 is a schematic block diagram of the circuit board of FIG.
9;
FIG. 11 is a schematic block diagram in cross-sectional view of a
megasonic system according to an embodiment of the present
disclosure in which sonic energy is coupled to a jet of fluid that
is then directed at a target workpiece;
FIG. 12 is a top plan view of a close packed, hexagonal array of
MEMS megasonic transducers according to an embodiment of the
present disclosure;
FIG. 13 is a perspective view of a square array of MEMS megasonic
transducers according to another embodiment of the present
disclosure;
FIG. 14 is a flowchart illustrating a method of cleaning a
workpiece using a MEMS transducers according to an embodiment of
the present disclosure;
FIG. 15 is a schematic block diagram illustrating an embodiment of
method for applying a drive voltage having a fixed phase delay to
focus sonic energy on a target workpiece;
FIG. 16 is a schematic block diagram illustrating an embodiment of
method for applying a drive voltage having a variable phase delay
to sweep sonic energy across the surface of the target
workpiece;
FIGS. 17A-G are schematic block diagrams illustrating a process for
fabricating MEMS transducers according to an embodiment of the
present disclosure; and
FIG. 18 is a schematic block diagram in plan view illustrating
another process for fabricating MEMS transducers according to
another embodiment of the present disclosure.
DETAILED DESCRIPTION
The present invention is directed to megasonic systems including
MicroElectroMechanical System (MEMS) transducers, and to methods of
fabricating and using the same.
A megasonic systems and methods according to the present invention
will now be described with reference to FIGS. 1 through 16. For
purposes of clarity, many of the details of megasonic systems in
general and megasonic cleaning systems in particular that are
widely known and are not relevant to the present invention have
been omitted from the following description. The drawings described
are only schematic and are non-limiting. In the drawings, the size
of some of the elements may be exaggerated and not drawn to scale
for illustrative purposes. The dimensions and the relative
dimensions may not correspond to actual reductions to practice of
the invention.
Referring to FIG. 1, in one exemplary embodiment the megasonic
system 100 includes a number of a voltage controlled
Micro-Electromechanical System (MEMS) transducers arranged in an
array on a MEMS die 102. Each individual MEMS transducer (not shown
in this figure) includes a movable membrane with a membrane
electrode coupled to a first potential, said membrane disposed
above and spaced apart from an upper surface of a die including a
cavity electrode coupled to a second potential. The MEMS die 102 is
physically attached to a circuit board or printed circuit board
(PCB 104), and electrically coupled through the PCB to a driver 106
adapted to provide a drive signal or drive voltage between the
membrane electrodes and cavity electrodes at a frequency selected
to operate the MEMS transducers at a predetermined megasonic
resonant frequency. The system 100 further includes a chuck 108 on
which a target workpiece 110, such as substrate or wafer, is
positioned proximal to the MEMS transducers, and a fluid 112 to
couple sonic energy 114 from the MEMS transducers to the target
workpiece. The fluid 112 can be contained in a bath or reservoir
(not shown in this figure) in which both the MEMs die 102 and
target workpiece 110 are immersed or exposed, or can include a jet
or stream of fluid to which sonic energy is coupled that is then
directed at the target workpiece. The fluid 112 can include an
aqueous or organic compound, such as water or other solvents
depending on the target workpiece 110 and the type of contamination
to be removed.
Optionally, as in the embodiment shown the megasonic system 100
further includes a stiff, mechanical backing plate 116 to which the
PCB 104 is attached, substantially enclosing the driver 106 and a
back surface of the PCB. The backing plate 116 provides rigidity
and flatness to the PCB 104 and MEMS die 102, which is desirable
for maintaining a front or top surface of the MEMS die within close
proximity to the target workpiece 110 to increase cleaning
efficiency without risk of damage to either the target workpiece or
the MEMS die. By close proximity it is meant a distance of about
100 micrometers (.mu.m) or less.
A MEMS transducer will now be described with reference to FIG. 2.
Referring to FIG. 2, a MEMS transducer 200 generally includes a
movable membrane 202 suspended above an upper surface of a MEMS die
204 by a plurality of support structures 206, such as posts or
sidewalls of a cavity 208 formed in the die. In addition to the
aforementioned membrane electrode, the movable membrane 202
includes a number of taut layers of dielectric material surrounding
or enclosing the membrane electrode. For example, in the embodiment
shown the movable membrane 202 includes a membrane electrode 210
including one or more layers of a high-temperature conductive
material, such as polysilicon (poly), tungsten (W) or
tungsten-silicide (WiSi.sub.2), overlying a bottom silicon nitride
(Si.sub.3N.sub.4) layer 212, and overlaid by top silicon nitride
layer 214. Although, the intrinsic stress of a polysilicon membrane
electrode 210 is approximately zero, the intrinsic stress of the
silicon nitride layers 212, 214 is high, on the order of about 1
gigapascal (GPa) or more, sufficient to provide a substantially
flat, elastic movable membrane 202. The membrane electrode 210 is
electrically coupled to a first potential by one or more vertical
contacts 216 extending through the MEMS die 204 to a backside or
surface thereof. As noted above, the MEMS transducer 200 further
includes a bottom or cavity electrode 218 towards which the
membrane electrode can be attracted to move the movable membrane
202. The cavity electrode 214 also includes a high-temperature
conductive material, such as polysilicon (poly), tungsten (W) or
tungsten-silicide (WiSi.sub.2), and is electrically coupled to a
second potential a local interconnect (LM1) formed on or over a
surface of the MEMS die 204, and/or one or more vertical contacts
220 extending through the MEMS die. Generally, the cavity electrode
218 is over a first dielectric layer 222, such as an oxide or
silicon dioxide, formed on the surface of the MEMS die 204 and
covered by a second dielectric layer 224, such as an oxide or
silicon dioxide.
A significant advantage of the MEMS transducers of the present
invention over conventional piezoelectric transducers is the
ability to fabricate a large number of MEMS transducers in a close
packed array, or number of arrays on a single MEMS die. MEMS or
membrane densities of up to about 10.sup.4 membranes per cm.sup.2
can be readily achieved using current MEMS fabrication techniques.
This enables a megasonic system of the present invention to provide
much higher power densities, as we as a more uniform or controlled
distribution of sonic energy. Referring to FIG. 3, in one exemplary
embodiment the membrane or MEMS array 300 includes a single large,
10.times.40 mm, 4 cm.sup.2 array of about 4.times.10.sup.4
membranes. In the figure shown the surface of the MEMS array 300 is
filled with close packed membranes 302. In certain embodiments all
the membrane electrodes are shorted to each other and all the
cavity electrodes shorted to each other to provide low electrical
resistance, same phase operation and just two electrical
connections, power (V+) and ground (GND), thereby simplifying the
electrical connection to the PCB.
Optionally, in other embodiments, described in greater detail
below, the membrane electrodes and/or the cavity electrodes are not
all shorted to each other, but are coupled to individual MEMS
transducers or groups of MEMS transducers to enable a fixed or
variable phase delay to be applied to the drive signal, thereby
focusing or sweeping of the sonic energy.
The precise electrical voltage V+ or difference between the first
and second potentials required, as well as the maximum desired
frequency with which the MEMS transducers can be made to operate
will depend on gap between the membrane and the MEMS die surface,
as well as the physical parameters of the membranes themselves. A
schematic block diagram of a perspective view of a membrane of a
MEMS transducer is shown in FIG. 4. Referring to FIG. 4, it is has
been found that where the membrane includes a substantially
circular surface, a diameter (2R) of from about 3 to about 100
.mu.m will provide the MEMS transducers with a predetermined
resonant frequency in a range of from about 3 to about 10 megahertz
(MHz). It has further been found that, depending on the diameter of
the membrane, operating voltages can be kept to less than about 50V
for gaps or between 0.4 to about 0.8 .mu.m. Additional membrane
parameters of exemplary embodiments are summarized in Table I
below.
TABLE-US-00001 TABLE I 30 um 40 um 50 um 80 um 100 um Radius um 15
20 30 40 50 Membrane um 0.10 0.12 0.16 0.18 0.20 thickness Linear
um 0.10 0.13 0.15 0.18 0.20 displacement Gap thickness um 0.40 0.50
0.60 0.70 0.80 Stress GPa 1.0 1.0 1.0 1.0 1.0 Si3N4 modulus GPa 270
270 270 270 270 Poissan ratio -- 0.25 0.25 0.25 0.25 0.25 Biaxial
modulus Pa 360 360 360 360 360 Density kg/m.sup.3 3440 3440 3440
3440 3440 Drive voltage V 44 50 50 50 51 Stiffness N/m 658 785 1034
1155 1278 Force N 6.5E-05 9.6E-05 1.5E-04 2.0E-04 2.5E-04 Max
Energy pJ 3 6 11 17 25 Resonant MHz 11.4 8.6 5.7 4.3 3.4 Frequency
Power mW 0.04 0.05 0.06 0.07 0.09 (per resonator) Power density
W/cm.sup.2 3.8 3.0 1.7 1.1 0.8 Displacement pL 0.04 0.08 0.21 0.44
0.79 volume Effective mass kg 1.2E-13 2.6E-13 7.8E-13 1.6E-12
2.7E-12 Velocity m/s 1.1 1.1 0.9 0.8 0.7
In certain embodiments, it is desirable that a top or membrane
surface of the MEMS die be placed in close proximity to the target
workpiece, and therefore more usual top surface electrical contacts
to the MEMS die cannot be used. Accordingly, in another aspect of
the present invention a method of forming electrical contacts that
do not extend substantially above a plane of the top surface of the
MEMS die is provided. Referring to FIG. 5, in one embodiment a MEMS
die 502 is electrically coupled to a driver (not shown) through a
number of silicon vias 504 extending through a PCB 506 to backside
pads 508 on a back surface of the PCB, thereby enabling a
separation of about 100 .mu.m, or less, to be maintained between a
top surface of the MEMS die and a target workpiece 510.
In another embodiment, shown in FIG. 6, a perimeter of a MEMS die
602 is etched, for example by using a potassium hydroxide (KOH) wet
etch, to enable the die to be electrically coupled a PCB (not shown
in this figure) by wire-bonds 604, thereby while still enabling a
separation of about 100 .mu.m to be maintained between a top
surface of the MEMS die and a target workpiece 606.
Referring to FIG. 7A, in yet another aspect the present invention
is directed to an array (not shown) of MEMS transducers 702
including an interconnect grid 704 for interconnecting the membrane
electrodes 706 of substantially all or groups of MEMS transducers
in the array. The interconnect grid 704 includes a high-temperature
conductive material, such as polysilicon (poly), tungsten (W) or
tungsten-silicide (WiSi.sub.2). In addition to simplifying
electrical connections to the MEMS die 708, as noted above,
interconnecting the membrane electrodes 706 reduces resistance and
allows for `ganged` operation of the substantially all or groups of
MEMS transducers 702. Use of the interconnect grid 704, as opposed
to an interconnect layer, and careful selection of the thickness
and permittivity of a dielectric layer 710 separating the
interconnect grid 704 from a lower or cavity electrode 712 can
substantially eliminate capacitive coupling with the cavity
electrode that might otherwise limit the maximum achievable
frequency of the MEMS transducers 702. In particular, it has been
found that an array having a 75% fill ration, and having a 6 k.ANG.
thick, interconnect grid 704 covering about 10% surface and having
a corner to center (of the interconnect grid) resistance of about 1
Ohm, and with an estimated capacitive coupling through a 1 .mu.m
oxide dielectric layer 710 of about 10 nanoFarads (nF), megasonic
oscillations of up to about 5 MHz can still be achieved with a
displacement current of 2.5 amperes (A) or less. FIG. 7B is a
cross-sectional view of a portion of the interconnect grid 704 of
FIG. 7A. FIG. 8 is a schematic diagram in plan view of an
embodiment of an interconnect 802. Additional dimensions and RC
considerations of the above described exemplary embodiment of the
interconnect grid are summarized in Table II below.
TABLE-US-00002 TABLE II X Dimension mm 10 Y Dimension mm 40 Bus
width X mm 1 Bus width Y mm 4 Squares to center squares 20.0 M1
Thickness A 6000 Resistivity ohm-cm 3.0E-06 Sheet resistance
ohm/square 5.0E-02 M1 Resistance ohm 1.0 Dielectric Thickness um
1.0 Dielectric constant -- 4.0 % overlap % 0.70 Capactiance nF 10.0
Operating voltage V 50 Charge C 5.0E-07 RC timeconstant ns 199.4
Frequency MHz 5.0 Surge current A 2.50
Generally to provide the most complete and uniform cleaning of a
target workpiece, it is desirable to provide a relative motion
between the target workpiece and the MEMS transducers. Referring to
FIGS. 9 and 10, in one embodiment wherein the target workpiece 900
is, for example, a substrate or semiconductor wafer, this can be
accomplished by providing a PCB 902 with multiple drivers 904 and
multiple MEMS die 906, each with one or more arrays of MEMS
transducers. Generally, as in the embodiment show, the PCB 902
extends radially outward from a center of a rotating target
workpiece 900. FIG. 10 is a schematic block diagram of the PCB 902
of MEMS die 906 of FIG. 9. Referring to FIG. 10, in the embodiment
shown the drivers 904 are attached or mounted on a first or back
surface of the PCB 902 while multiple overlapping MEMS die 906
(shown in phantom with dashed lines) are attached or mounted on a
second or front surface of the PCB.
In yet another embodiment, shown in FIG. 11, the megasonic system
1100 can include one or more nozzles 1102 in which sonic energy
1104 from a MEMS die 1106 is coupled to a jet or stream of fluid
1108, which is then directed on to a target workpiece 1110
positioned or held on a chuck 1112.
Although the exemplary embodiments of the MEMS arrays described
heretofore have included arrays having a rectangular shape, it will
be appreciated that this need not be the case, and the MEMS
transducers can be located on the die to form a triangular, square,
hexagonal or other polygonal shaped array. In particular, FIG. 12
is a top plan view of a close packed, hexagonal array 1200 of MEMS
megasonic transducers 1202 according to an embodiment of the
present disclosure. FIG. 13 is a perspective view of a MEMS die
1300 including a square array 1302 of MEMS megasonic transducers
1304 according to another embodiment of the present disclosure.
A method for cleaning a workpiece using an array of megasonic MEMS
transducers described above will now be described with reference to
the flowchart of FIG. 14.
Referring to FIG. 14, the method begins with providing a plurality
of MEMS transducers, each including a movable membrane with a
membrane electrode coupled to a first potential disposed above and
spaced apart from an upper surface of a die including a cavity
electrode coupled to a second potential. (Step 1400) A target
workpiece is then positioned on a chuck proximal to the MEMS
transducers. (Step 1402) Next, a drive voltage is applied between
the membrane electrodes and cavity electrodes to operate the MEMS
transducers at a megasonic resonant frequency (Step 1404), and
coupling sonic energy to the target workpiece from the MEMS
transducers through a fluid. (Step 1406)
Optionally, a phase of the drive voltage is applied to a first
number or group of MEMS transducers can be varied radially relative
to that supplied to a second number or group of MEMS transducers to
achieve phased-array focusing of sonic energy emitted from the MEMS
transducers on the target workpiece. (Step 1408) An example of this
embodiment is shown in FIG. 15. Referring to FIG. 15, the phase of
a drive voltage applied to one or more MEMS transducers 1502 near a
center 1504 of an array 1506 can be delayed with a fixed phase
delay relative to other MEMS transducers near an outer edge 1508 of
the array to generate a peak of sonic energy that focuses sonic
energy on a target workpiece.
Alternatively or additionally, the phase of the drive voltage is
applied to a first number of MEMS transducers can be temporally
varied relative to that supplied to the second number of MEMS
transducers to sweep sonic energy emitted from the MEMS transducers
across a surface of the target workpiece. (Step 1410) For example,
referring to FIG. 16 the drive voltage can be applied to one or
more consecutive rows 1602 of MEMS transducers 1604 in a
rectangular array 1606 can be delayed relative to other rows of to
generate a peak of sonic energy (represented by arrow 1608) that
moves sequentially along an axis 1610 of the array perpendicular to
the rows to sweep sonic energy across the surface of the target
workpiece.
An embodiment of a method or process for fabricating megasonic MEMS
transducers according to the present invention will now be
described with reference to FIGS. 17A-G. FIGS. 17A-G illustrate
several sectional side views of after a number of processing steps,
the end of result of which is to produce a MEMS transducer similar
to those shown in FIGS. 2 and 7A. One process sequence is as
follows.
Referring to FIG. 17A, a lower or cavity electrode 1702 is formed
on or over a surface of a semiconductor substrate 1704, such as a
silicon wafer, and a dielectric layer 1706 formed thereover.
Although in the embodiment shown the cavity electrode 1702 is
formed directly on the surface of the semiconductor substrate 1704,
it will be understood that the cavity electrode can also be formed
on a dielectric layer formed over the surface of the semiconductor
substrate. This embodiment is particularly useful in those
embodiments where CMOS or other semiconductor circuits, such as a
driver, are integrally formed in the substrate prior to or
concurrently with the fabrication of the MEMS transducers. The
cavity electrode 1702 can include a high-temperature metal, such as
tungsten or tungsten-silicide (WiSi.sub.2), deposited by physical
vapor deposition, or a polysilicon deposited by chemical vapor
deposition (CVD). The dielectric layer 1706 can include an oxide,
such as a silicon dioxide, deposited by CVD or thermally grown.
Referring to FIG. 17B, an interconnect grid 1708 is formed on or
over the dielectric layer 1706 and a sacrificial layer 1710 formed
over the interconnect grid and dielectric layer. The interconnect
grid 1708 can be formed by patterning a layer of conducting
material using standard photolithographic techniques. The
conducting material can include metal, such as tungsten or
tungsten-silicide (WiSi.sub.2), deposited by chemical or physical
vapor deposition, or a polysilicon deposited by CVD. The
sacrificial layer 1710 can include amorphous silicon deposited by
CVD. By standard photolithographic techniques it is meant a process
using a light-sensitive photoresist followed by a number of
developing, wet or dry etching and resist stripping process to
selectively remove parts of the underlying layer(s) to transfer a
pattern thereto.
Referring to FIG. 17C, the sacrificial layer 1710 is patterned
using standard photolithographic techniques, a silicon nitride
layer 1712 deposited using low pressure chemical vapor deposition
(LPCVD) or plasma enhanced chemical vapor deposition (PECVD) to
form a number of support structures and a first or bottom silicon
nitride layer of a multi-layer membrane. Preferably, the silicon
nitride layer 1712 is deposited under conditions selected to
produce a taut silicon nitride having a high intrinsic stress
sufficient to provide a substantially flat, elastic movable
membrane.
Referring to FIG. 17D, the silicon nitride layer 1712 is patterned
using standard photolithographic techniques, and a metal deposited
therein to form an electrical contact 1714 to electrically couple
the interconnect grid 1708 to a membrane electrode 1716 formed on
the top surface of the silicon nitride layer 1712. The membrane
electrode 1716 can include metal, such as tungsten or
tungsten-silicide (WiSi.sub.2), deposited by chemical or physical
vapor deposition, or a polysilicon layer deposited by CVD.
Referring to FIG. 17E, a second silicon nitride layer 1724
deposited to form a top silicon nitride layer of a multi-layer
membrane 1722. As with the first silicon nitride layer 1712, the
second silicon nitride layer 1724 can be deposited by LPCVD or
PECVD under conditions selected to produce a taut silicon nitride
having a high intrinsic stress sufficient to provide a
substantially flat, elastic movable membrane 1722.
Referring to FIG. 17F, a number of openings 1718 are etched through
the second silicon nitride layer 1724, the membrane electrode 1716
and first silicon nitride layer 1712 to expose a portion of the
sacrificial layer 1710, and the sacrificial layer removed to form a
cavity 1720 and release the movable membrane 1722. In one
embodiment, in which the sacrificial layer 1710 includes amorphous
silicon, it is removed using Xenon difluoride (XeF.sub.2) chemistry
with a high selectivity to the silicon nitride layer 1712, the
dielectric layer 1706, and the material of the membrane electrode
1716.
Referring to FIG. 17G, the openings 1718 are sealed by deposition
of a thin layer or small amount of lugging material 1725. Suitable
lugging materials include aluminum (Al) deposited by sputter
deposition in an argon (Ar) environment. Finally, one or more
vertical contacts 1726 are formed extending through a backside or
surface of the substrate 1704 to electrically couple to the cavity
electrodes 1702 and to the interconnect grid 1708.
FIG. 18 illustrates another process for fabricating MEMS
transducers according to another embodiment of the present
disclosure. Referring to FIG. 18, in this embodiment, movable
membranes 1802 are supported above a cavity (not shown in this
figure) by a plurality of support structures 1804 having a number
of openings or passages 1806 there through so that the sacrificial
layer (not shown in this figure) can be removed through openings
1808 located between the movable membranes, thereby avoiding the
need to form openings extending through the movable membranes. As
with the openings 1718 of FIG. 17E, the openings 1808 can be sealed
by deposition of a lugging material (not shown in this figure).
Thus, embodiments of megasonic systems including MEMS transducers
and methods of fabricating and using the same have been described.
Although the present disclosure has been described with reference
to specific exemplary embodiments, it will be evident that various
modifications and changes may be made to these embodiments without
departing from the broader spirit and scope of the disclosure.
Accordingly, the specification and drawings are to be regarded in
an illustrative rather than a restrictive sense.
The Abstract of the Disclosure is provided to comply with 37 C.F.R.
.sctn.1.72(b), requiring an abstract that will allow the reader to
quickly ascertain the nature of the technical disclosure. It is
submitted with the understanding that it will not be used to
interpret or limit the scope or meaning of the claims. In addition,
in the foregoing Detailed Description, it can be seen that various
features are grouped together in a single embodiment for the
purpose of streamlining the disclosure. This method of disclosure
is not to be interpreted as reflecting an intention that the
claimed embodiments require more features than are expressly
recited in each claim. Rather, as the following claims reflect,
inventive subject matter lies in less than all features of a single
disclosed embodiment. Thus, the following claims are hereby
incorporated into the Detailed Description, with each claim
standing on its own as a separate embodiment.
In the forgoing description, for purposes of explanation, numerous
specific details have been set forth in order to provide a thorough
understanding of the system and method of the present disclosure.
It will be evident however to one skilled in the art that the
present interface device and method may be practiced without these
specific details. In other instances, well-known structures, and
techniques are not shown in detail or are shown in block diagram
form in order to avoid unnecessarily obscuring an understanding of
this description.
Reference in the description to "one embodiment" or "an embodiment"
means that a particular feature, structure, or characteristic
described in connection with the embodiment is included in at least
one embodiment of the system or method. The appearances of the
phrase "one embodiment" in various places in the specification do
not necessarily all refer to the same embodiment. The term "to
couple" as used herein may include both to directly electrically
connect two or more components or elements and to indirectly
connect through one or more intervening components.
* * * * *